diff --git a/src/buffer.c b/src/buffer.c
index d6d039f..355dbc7 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1747,7 +1747,8 @@
     buf->b_wininfo->wi_win = curwin;
 
 #ifdef FEAT_EVAL
-    init_var_dict(&buf->b_vars, &buf->b_bufvar);    /* init b: variables */
+    /* init b: variables */
+    init_var_dict(&buf->b_vars, &buf->b_bufvar, VAR_SCOPE);
 #endif
 #ifdef FEAT_SYN_HL
     hash_init(&buf->b_s.b_keywtab);
diff --git a/src/eval.c b/src/eval.c
index 5fc32e2..99b3bba 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -850,8 +850,8 @@
     int		    i;
     struct vimvar   *p;
 
-    init_var_dict(&globvardict, &globvars_var);
-    init_var_dict(&vimvardict, &vimvars_var);
+    init_var_dict(&globvardict, &globvars_var, VAR_DEF_SCOPE);
+    init_var_dict(&vimvardict, &vimvars_var, VAR_SCOPE);
     vimvardict.dv_lock = VAR_FIXED;
     hash_init(&compat_hashtab);
     hash_init(&func_hashtab);
@@ -2725,14 +2725,26 @@
 	    lp->ll_dict = lp->ll_tv->vval.v_dict;
 	    lp->ll_di = dict_find(lp->ll_dict, key, len);
 
-	    /* When assigning to g: check that a function and variable name is
-	     * valid. */
-	    if (rettv != NULL && lp->ll_dict == &globvardict)
+	    /* When assigning to a scope dictionary check that a function and
+	     * variable name is valid (only variable name unless it is l: or
+	     * g: dictionary). Disallow overwriting a builtin function. */
+	    if (rettv != NULL && lp->ll_dict->dv_scope != 0)
 	    {
-		if (rettv->v_type == VAR_FUNC
+		int prevval;
+		int wrong;
+
+		if (len != -1)
+		{
+		    prevval = key[len];
+		    key[len] = NUL;
+		}
+		wrong = (lp->ll_dict->dv_scope == VAR_DEF_SCOPE
+			       && rettv->v_type == VAR_FUNC
 			       && var_check_func_name(key, lp->ll_di == NULL))
-		    return NULL;
-		if (!valid_varname(key))
+			|| !valid_varname(key);
+		if (len != -1)
+		    key[len] = prevval;
+		if (wrong)
 		    return NULL;
 	    }
 
@@ -6951,7 +6963,7 @@
     d = (dict_T *)alloc(sizeof(dict_T));
     if (d != NULL)
     {
-	/* Add the list to the list of dicts for garbage collection. */
+	/* Add the dict to the list of dicts for garbage collection. */
 	if (first_dict != NULL)
 	    first_dict->dv_used_prev = d;
 	d->dv_used_next = first_dict;
@@ -6960,6 +6972,7 @@
 
 	hash_init(&d->dv_hashtab);
 	d->dv_lock = 0;
+	d->dv_scope = 0;
 	d->dv_refcount = 0;
 	d->dv_copyID = 0;
     }
@@ -10203,6 +10216,19 @@
 		{
 		    --todo;
 		    di1 = dict_find(d1, hi2->hi_key, -1);
+		    if (d1->dv_scope != 0)
+		    {
+			/* Disallow replacing a builtin function in l: and g:.
+			 * Check the key to be valid when adding to any
+			 * scope. */
+		        if (d1->dv_scope == VAR_DEF_SCOPE
+				&& HI2DI(hi2)->di_tv.v_type == VAR_FUNC
+				&& var_check_func_name(hi2->hi_key,
+								 di1 == NULL))
+			    break;
+			if (!valid_varname(hi2->hi_key))
+			    break;
+		    }
 		    if (di1 == NULL)
 		    {
 			di1 = dictitem_copy(HI2DI(hi2));
@@ -20027,7 +20053,7 @@
 	{
 	    sv = SCRIPT_SV(ga_scripts.ga_len + 1) =
 		(scriptvar_T *)alloc_clear(sizeof(scriptvar_T));
-	    init_var_dict(&sv->sv_dict, &sv->sv_var);
+	    init_var_dict(&sv->sv_dict, &sv->sv_var, VAR_SCOPE);
 	    ++ga_scripts.ga_len;
 	}
     }
@@ -20038,12 +20064,14 @@
  * point to it.
  */
     void
-init_var_dict(dict, dict_var)
+init_var_dict(dict, dict_var, scope)
     dict_T	*dict;
     dictitem_T	*dict_var;
+    int		scope;
 {
     hash_init(&dict->dv_hashtab);
     dict->dv_lock = 0;
+    dict->dv_scope = scope;
     dict->dv_refcount = DO_NOT_FREE_CNT;
     dict->dv_copyID = 0;
     dict_var->di_tv.vval.v_dict = dict;
@@ -22304,7 +22332,7 @@
     /*
      * Init l: variables.
      */
-    init_var_dict(&fc->l_vars, &fc->l_vars_var);
+    init_var_dict(&fc->l_vars, &fc->l_vars_var, VAR_DEF_SCOPE);
     if (selfdict != NULL)
     {
 	/* Set l:self to "selfdict".  Use "name" to avoid a warning from
@@ -22325,7 +22353,7 @@
      * Set a:0 to "argcount".
      * Set a:000 to a list with room for the "..." arguments.
      */
-    init_var_dict(&fc->l_avars, &fc->l_avars_var);
+    init_var_dict(&fc->l_avars, &fc->l_avars_var, VAR_SCOPE);
     add_nr_var(&fc->l_avars, &fc->fixvar[fixvar_idx++].var, "0",
 				(varnumber_T)(argcount - fp->uf_args.ga_len));
     /* Use "name" to avoid a warning from some compiler that checks the
diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index 2454f2a..63c140c 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -93,7 +93,7 @@
 char_u *get_tv_string_chk __ARGS((typval_T *varp));
 char_u *get_var_value __ARGS((char_u *name));
 void new_script_vars __ARGS((scid_T id));
-void init_var_dict __ARGS((dict_T *dict, dictitem_T *dict_var));
+void init_var_dict __ARGS((dict_T *dict, dictitem_T *dict_var, int scope));
 void vars_clear __ARGS((hashtab_T *ht));
 void copy_tv __ARGS((typval_T *from, typval_T *to));
 void ex_echo __ARGS((exarg_T *eap));
diff --git a/src/structs.h b/src/structs.h
index abcb704..36440c3 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1106,6 +1106,11 @@
 #define VAR_DICT    5	/* "v_dict" is used */
 #define VAR_FLOAT   6	/* "v_float" is used */
 
+/* Values for "dv_scope". */
+#define VAR_SCOPE     1	/* a:, v:, s:, etc. scope dictionaries */
+#define VAR_DEF_SCOPE 2	/* l:, g: scope dictionaries: here funcrefs are not
+			   allowed to mask existing functions */
+
 /* Values for "v_lock". */
 #define VAR_LOCKED  1	/* locked with lock(), can use unlock() */
 #define VAR_FIXED   2	/* locked forever */
@@ -1181,6 +1186,7 @@
     int		dv_copyID;	/* ID used by deepcopy() */
     dict_T	*dv_copydict;	/* copied dict used by deepcopy() */
     char	dv_lock;	/* zero, VAR_LOCKED, VAR_FIXED */
+    char	dv_scope;	/* zero, VAR_SCOPE, VAR_DEF_SCOPE */
     dict_T	*dv_used_next;	/* next dict in used dicts list */
     dict_T	*dv_used_prev;	/* previous dict in used dicts list */
 };
diff --git a/src/testdir/test34.in b/src/testdir/test34.in
index 28fd00c..5abc140 100644
--- a/src/testdir/test34.in
+++ b/src/testdir/test34.in
@@ -1,5 +1,6 @@
 Test for user functions.
 Also test an <expr> mapping calling a function.
+Also test that a builtin function cannot be replaced.
 
 STARTTEST
 :so small.vim
@@ -58,7 +59,10 @@
 ---*---
 (one
 (two
-[(one again:$-5,$w! test.out
+[(one again:call append(line('$'), max([1, 2, 3]))
+:call extend(g:, {'max': function('min')})
+:call append(line('$'), max([1, 2, 3]))
+:$-7,$w! test.out
 :delfunc Table
 :delfunc Compute
 :delfunc Expr1
diff --git a/src/testdir/test34.ok b/src/testdir/test34.ok
index d71183d..6053812 100644
--- a/src/testdir/test34.ok
+++ b/src/testdir/test34.ok
@@ -4,3 +4,5 @@
 1. one
 2. two
 1. one again
+3
+3
diff --git a/src/version.c b/src/version.c
index 4e17878..6bf9708 100644
--- a/src/version.c
+++ b/src/version.c
@@ -715,6 +715,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    603,
+/**/
     602,
 /**/
     601,
diff --git a/src/window.c b/src/window.c
index 6460684..c0b7b8b 100644
--- a/src/window.c
+++ b/src/window.c
@@ -3468,7 +3468,7 @@
 # endif
 #ifdef FEAT_EVAL
 	/* init t: variables */
-	init_var_dict(&tp->tp_vars, &tp->tp_winvar);
+	init_var_dict(&tp->tp_vars, &tp->tp_winvar, VAR_SCOPE);
 #endif
 	tp->tp_ch_used = p_ch;
     }
@@ -4410,7 +4410,7 @@
 #endif
 #ifdef FEAT_EVAL
 	/* init w: variables */
-	init_var_dict(&new_wp->w_vars, &new_wp->w_winvar);
+	init_var_dict(&new_wp->w_vars, &new_wp->w_winvar, VAR_SCOPE);
 #endif
 #ifdef FEAT_FOLDING
 	foldInitWin(new_wp);